The present disclosure relates generally to a magnetic field generating device and method of shimming thereof, and particularly to a magnetic field generating device for use in magnetic resonance imaging (MRI).
Subjecting a substance such as a human tissue to a uniform magnetic field (a polarizing field) B0 in the z-direction, not only results in the individual magnetic moments of the spins in the tissue attempting to align themselves with the polarizing field, but also results in the moments precessing about the B0 field in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a transverse magnetic field (excitation field) B1 in the x-y plane that is near the Larmor frequency, the net aligned moment, or longitudinal magnetization Mz, may be rotated or tipped into the x-y plane to produce a net transverse magnetic moment Mt. After the excitation signal B1 is terminated, a signal is emitted by the relaxation of the excited spins that may be received and processed to form an image. When utilizing these emitted signals (nuclear magnetic resonant, NMR, signals) to produce images, magnetic field gradients (Gx, Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many reconstruction techniques.
For useful imaging, MRI magnets used in a magnetic resonant assembly not only require an intensive uniform magnetic field, typically less than 10 ppm (parts per million) of field variation within a typical 30-50 centimeter (cm) spherical volume, but also require an accurate center magnetic field value, typically less than 0.5% variation. Other magnets useful in MRI include extremity magnets, which have a substantially smaller field of view (FOV). The magnetic field of an as-manufactured magnet is often influenced by the deviation of material properties and the tolerance of manufacturing processes, resulting in a very inhomogeneous field ranging from several hundred to several thousand ppm field variation, and a non-accurate center magnetic field that is far from the desired value. To obtain the desired accuracy of center magnetic field and the desired uniformity of magnetic field in the image volume for useful imaging, it is necessary to adjust the center magnetic field and to reduce the field inhomogeneity to desired values. For a permanent magnet (PM) design absent a poleface, that is, a PM absent a surface having the function of shifting the direction of the local field lines, a shim holder is typically employed for holding a PM shim that is used for adjusting the B0 field and homogeneity. The use of a shim holder may be costly, inconvenient and space inefficient, thereby consuming precious patient gap at a high cost.
Accordingly, there is a need in the MRI art for a magnetic field generating device and method of shimming thereof that overcomes these drawbacks.